1. Field of the Invention
The present invention relates to a method and evaporation chamber for generating a continuous vapor stream containing a compound in which gallium is present in monovalent form in a vacuum coating process for vacuum coating a substrate. The invention also relates to a vacuum coating apparatus operating according to such a method and having such an evacuation chamber.
2. Description of the Prior Art
Monovalent gallium (Ga.sup.1+) is frequently used as a doping substance in X-ray absorber materials such as CsBr or RbBr for storage phosphors. Substrates coated therewith are needed as radiation detectors for radiographic applications, for example. The monovalent gallium is applied as GaBr, but GaBr cannot be chemically isolated. To achieve a doping despite this fact, it is suggested in U.S. Pat. No. 5,736,069 to use GaCl as the starting material, which is mixed with metallic Ga and with the X-ray absorber material CsBr. This mix is dried at 400.degree. C., after which a monocrystal of the phosphor is grown at high temperature (e.g. 925.degree. C.). The powder-type phosphor is obtained by milling the monocrystal, and is dispersed in a solvent (e.g. ethyl acetate) in a solution of a binder (e.g. polyethyl acrylate), and is layered onto the carrier film, for instance by pouring, rolling or sedimenting. High-quality layers cannot be produced in this way. Good X-ray absorber layers are usually produced in the context of a vacuum evaporating process; that is, the X-ray absorber material is first evaporated in an evaporating device and then settles on the carrier. The high vapor pressure of the required starting materials, namely the bivalent or trivalent gallium-halogenide compound and the gallium metal, acts to inhibit deposition of the doping substance containing monovalent gallium, which can be GaBr or GaI, by means of a vacuum coating method, since these would evaporate abruptly in the vacuum at common evaporating temperatures in the range of a few hundred degrees upon the formation of the dopant, even before the reaction of the starting materials has begun. As a result, it has not been possible to perform doping with monovalent gallium in the context of a vacuum coating, and particularly to simultaneously generate the X-ray absorber layer and to introduce the doping in the context of an evaporation process.